An injection-molding machine generally comprises a pair of mold-carrying plates each provided with a so-called mold half, the mold halves defining at least one cavity therebetween. The machine can be provided with an injection ram or other means for forcing, at an elevated pressure, a plastified synthetic-resin mass into the mold cavity formed between the mold halves in a closed position of the mold.
At least one of the mold-carrying plates is generally movable, i.e. can be displaced toward and away from the other plate in a mold-closing and mold-opening direction, respectively. For this purpose it is known to provide a mold-closing device which can be hydraulically actuated.
For example, it is known to provide a double-acting main piston which is coupled with the movable mold-carrying plate and which is shiftable in a cylinder which defines with the main piston an opening-side compartment and a closing-side compartment. The opening-side compartment is the compartment which is turned away from the mold while the closing-side compartment is proximal to the mold. The opening-side compartment is generally pressurized for mold-closing movement of the main piston while the closing-side compartment may be pressurized for mold-closing movement of the piston.
In addition, the movable mold plate can be provided with a rapid-action piston-and-cylinder arrangement for rapid-opening and rapid-closing movements of the movable mold plate. To allow this rapid-action hydraulic device to be effective, the main piston can be provided with a closable passage communicating between its main compartments.
Thus, when only the sliding friction of the mold plate and its assembly for opening and closing movements must be overcome, the rapid-action hydraulic unit is effective and is not impeded by the main piston-and-cylinder arrangement because of the communication between the two compartments thereof through the closable passage in the main cylinder.
When, however, greater force must be applied, for example, in the final mold-closing stage, to retain the mold closed against the development of considerable pressure in the mold cavity during the injection step, and to break open the mold after injection of the article, the passage between the main compartments is closed and the main compartments are selectively pressurized to develop considerable force in the mold-opening or mold-closing directions as required.
A mold-closing device of this type is described in German patent document (open application--Offenlegungsschrift) DE OS No. 1,778,952. In this system the cylinder chambers or compartments fore and aft of the main piston are connected by a passage whose purpose is to minimize the quantity of fluid which must be displaced by the main piston. This system has, however, the disadvantage that a special valve must be provided to close this passage when the main cylinder and its piston are to be effective, this valve requiring special controls. Furthermore, frequently the system pressure to which the mold-closing device is connected will require dimensioning the main piston for the requisite mold-closing or mold-opening force, so that the system is relatively expensive or unwieldy.
Mention may also be made of mold-closing systems using hydraulic principles as, for example, represented in German patent document (open application--Offenlegungsschrift) No. 2,432,638, which connect the main cylinder with a pressure-transmitting piston-and-cylinder arrangement. In this system, a significant disadvantage arises because additional hydraulic lines are required to connect the second cylinder with the first. Furthermore, a compact assembly of the two cannot be provided readily.
Note also the mold-closing system which is disclosed in German patent document (open application--Offenlegungsschrift) No. 2,423,806 and which appears to require a relatively complex main piston construction and, although it has an opening and closing device coupled therewith, cannot readily generate extremely high closing forces, except by a use of a relatively large main piston with the disadvantages already described.
A similar disadvantage appears to be present in the mold-closing device of German patent document (printed application--Auslegeschrift) No. 1,554,881.